North America has no records going as far back as the Medieval Warming, unless we count the oral testimony of the Greenland Vikings. The sagas say they found grapes growing on what is now called Newfoundland. The Vikings, impressed, named it “Vinland,” or vineland.

For the more recent Little Ice Age, fur traders’ records tell us that lake freezing dates now are 8.7 days later in the Hudson’s Bay region than they were in 1850, while ice break-up dates are 9.8 days earlier. That means temperatures are now about 1.8 degrees C warmer.

Ezra Stiles, then president of Yale University, recorded daily Connecticut temperatures from 1779. His records show June 1816 is still the coldest June ever recorded in the state, 2.5 degrees C colder than the mean for Connecticut from 1780 to 1968.

The two most broadly powerful pieces of climate cycle evidence in North America:

First, the North American Pollen Database reveals nine continent-wide temperature-driven shifts in vegetation during the past 14,000 years, an average of one every 1,650 years.^{92, 93} Thousands of pollen studies show that the vegetation shifts occurred across the whole of North America. The most recent major shift happened about 600 years ago “culminating in the Little Ice Age, with maximum cooling 300 years ago.” The previous shift began about 1,600 years ago, and “culminated in the maximum warming of the Medieval Warm Period 1,000 years ago.” The pollen analysis was led by Andre Viau of the University of Ottawa who wrote, “We suggest that North Atlantic millennial-scale climate variability is associated with rearrangements of the atmospheric circulation with far-reaching influences on the climate.”

Ian Campbell and John McAndrews of Environment Canada found that as the Little Ice Age cooled southern Ontario’s climate, the predominant forest trees shifted from warmth-loving beeches to cold-tolerant oaks, and then to cold-adapted evergreens.⁹⁴ Since 1850, that shift has been reversing, with oak trees gradually displacing the evergreens, and the beech trees awaiting their turn. The scientists’ computer simulation found the tree species still lagging behind the Modern Warming in 1993, though most of the warming predated 1940. Tree populations may thus need centuries to adapt to the major climate shifts. Campbell and McAndrews also concluded that the total plant mass of Ontario forests fell by 30 percent in the Little Ice Age; the forests still have not recovered the full productivity they had during the Medieval Warming.

Second, the water levels of the Great Lakes show a strong response to the 1,500-year climate cycle, with the lake levels high during climate coolings and low during warming periods. Todd Thompson of Indiana University and Steve Baedke of James Madison University studied “strandplains”— shore-parallel sand ridges that have a core of water-laid sediment.⁹⁵ These “strandplains” commonly occur on shore as a series of ridges and swales that reveal the upper level of the lake waters, while the organic sediments indicate the age of the ridges.

The Great Lakes were at their highest levels in the record — by far — about 4,500 years ago, just after the end of the very warm Climate Optimum. (The last of the huge Laurentide Ice Sheet may have melted during that period.) Water levels fell sharply — by 4.5 meters — during the next 1,000 years, and then rose, less dramatically, between 3,100 and 2,300 years ago during the cold that preceded the Roman Warming. The lakes were low from 2,300 to 1,900 years ago, reflecting the Roman Warming. Then water levels rose again from 100 to 900, in response to the cold Dark Ages. The waters were high again in both 1300 and 1600, reflecting the two-stage Little Ice Age.⁹⁶

Evidence of warmer past temperatures has also been found in Central Alaska — in the expansion of forest ranges, and the absence of permafrost during the prior interglacial era.⁹⁷

In Northern Quebec, centuries-old ice wedges in the soil surface indicate that the region was cold until about A.D. 140, followed by a warmer period that persisted up to 1030, the Medieval Warming. The region was severely cold between 1500 and 1900 — the Little Ice Age. Colder conditions have prevailed again during the last 50 years.⁹⁸

In the southern Sierra Nevada Mountains, foxtail pine and western juniper tree rings indicate a Medieval Warming from 1100 to 1375, and a cold period from 1450 to 1850, corresponding to the Little Ice Age.⁹⁹ Tree ring widths in the very long-lived bristlecone pine trees of the Sierra Nevada correlate statistically from 800 to the present century “with the temperatures derived for central England.”¹⁰⁰

Lisa Graumlich of Montana State University combined both tree rings and tree line changes in the Sierra Nevada, where high-growing trees are preserved in place, living and dead, for up to 3,000 years. Graumlich says, “A relatively dense forest grew above the current tree line from the beginning of our records to around 100 B.C., and again from A.D. 400 to 1000, when temperatures were warm. Abundance of trees and elevation of tree line declined very rapidly from 1000 to 1400, the period of severe, multi-decadal droughts. Tree lines declined more slowly from 1500 to 1900 under the cool temperatures of the Little Ice Age, reaching current elevations around 1900.”¹⁰¹